1. Field of the Invention
This invention relates to cyclic organosilicon compounds that may be employed as an electron donor for polymerization catalyst systems, to polymerization catalyst systems employing the cyclic organosilicon compounds as an electron donor, to methods of making the polymerization catalyst systems, and to polymerization processes to produce polyolefins, particularly polypropylene, having broadened molecular weight distribution employing the polymerization catalyst systems.
2. Description of the Related Art
Ziegler-Natta catalyst systems for polyolefin polymerization are well known in the art. Commonly, these systems are composed of a solid Ziegler-Natta catalyst component and a co-catalyst component, usually an organoaluminum compound. To increase the activity and sterospecificity of the catalyst system for the polymerization of α-olefins, electron donating compounds have been widely used (1) as an internal electron donor in the solid Ziegler-Natta catalyst component and/or (2) as an external electron donor to be used in conjunction with the solid Ziegler-Natta catalyst component and the co-catalyst component. Organosilicon compounds are commonly used as external electron donors.
Common internal electron donor compounds, incorporated in the solid Ziegler-Natta catalyst component during preparation of such component include ethers, ketones, amines, alcohols, phenols, phosphines, and silanes. Examples of such internal electron donor compounds and their use as a component of the catalyst system are described in U.S. Pat. Nos. 4,107,414; 4,186,107; 4,226,963; 4,347,160; 4,382,019; 4,435,550, 4,465,782; 4,522,930; 4,530,912; 4,532,313; 4,560,671; 4,657,882; 5,208,302; 5,902,765; 5,948,872; 6,121,483; and 6,770,586.
In the utilization of Ziegler-Natta type catalysts for polymerizations involving propylene or other olefins for which isotacticity is a possibility, it may be desirable to utilize an external electron donor, which may or may not be in addition to the use of an internal electron donor. It is known in the art that external electron donors act as stereoselective control agents to improve isotacticity, i.e., stereoregularity of the resulted polymer products, by selectively poisoning or converting the active site of non-stereoregularity present on the surface of a solid catalyst. Also, it is well known that polymerization activity, as well as stereoregularity and molecular weight and molecular weight distribution of the resulting polymer, depend on the molecular structure of external electron donor employed. Therefore, in order to improve polymerization process and the properties of the resulting polymer, there has been an effort and desire to develop various external electron donors, particularly various organosilane compounds. Examples of such external electron donors known in the art are organosilicon compounds containing Si—OCOR, Si—OR, or Si—NR2 bonds, having silicon as the central atom, where R is commonly an alkyl, alkenyl, aryl, arylalkyl, or cycloalkyl with 1-20 carbon atoms. Such compounds are described in U.S. Pat. Nos. 4,472,524; 4,473,660; 4,560,671; 4,581,342; 4,657,882; 5,106,807; 5,407,883; 5,684,173; 6,228,961; 6,362,124; 6,552,136; 6,689,849; 7,009,015; and 7,244,794.
WO03014167 uses cyclic organosilicon compounds containing hetero-atom as an external electron donor in a catalyst system to prepare polypropylene with higher melt flow rate (MFR). The silicon is embedded in a ring system, wherein only one hetero-atom is present. The propylene polymer prepared by using organosilane G with purity of 96% as an external electron donor is stated to have a narrow molecular weight distribution. No molecular weight distribution data are presented for the propylene polymers prepared using other pure organosilanes (purity>95%) as external electron donors.
For certain applications, polymers with a wider molecular weight distribution are desirable. Such polymers have a lower melt viscosity at high shear rates. Many polymer fabrication processes operating with high shear rates, such as injection molding, oriented film and thermobonded fibers, could benefit from a lower viscosity product by improving throughput rates and reducing energy costs. Products with higher stiffness, as measured by flexural modulus, are important for injection molded, extruded, and film products, as the fabricated parts can be down-gauged so that less material is needed to maintain product properties. Broad molecular weight distribution is one of the important contributors to achieving high stiffness of polymeric materials. Therefore, it can be advantageous to tailor polymerization catalyst systems to obtain polymers with a wider molecular weight distribution.
Methods are described in JP-A-63-245408, JP-A-2-232207, and JP-A-4-370103 for the preparation of polymers with wide molecular weight distribution obtained by polymerizing propylene in plural numbers of polymerization vessels or by multiple stage polymerizations. However, the disclosed operations are complicated with low production efficiency, and polymer structure and product quality are difficult to control.
There have been continuing efforts to tailor polymerization catalyst systems to enhance resin processability/extrusion characteristics via broadening of polymer molecular weight distribution through utilization of particular types of external electron donor systems. U.S. Pat. No. 6,376,628 teaches bis(perhydroisoquinolino)dimethoxysilane compounds and U.S. Pat. No. 6,800,703 teaches vinyltrimethoxysilane or dicyclohexyldimethoxysilane compounds as external electron donor to produce polypropylene with broad molecular weight distribution. U.S. Patent Application Publication 20060252894 discloses using mixed donor systems comprising different silane compounds to produce polypropylene with broadened molecular weight distribution.
There is a continuing need for developing catalyst systems that can be used to produce polyolefins, particularly polypropylene, with broadened molecular weight distribution. In addition to broadened molecular weight distribution, desired catalyst systems should also offer good polymerization activity and hydrogen response. Furthermore, the catalyst systems should also offer a steady and wide operating window for controlling isotacticity of the resulting polymers based on end user application requirement.